The present invention relates generally to the formation of beryllium oxide articles by a pressure-sintering operation and, more particularly, to the method of forming such articles wherein a sintering aid of lithium in an improved form is employed for facilitating the sintering of beryllium oxide particulates.
The formation of beryllium oxide (BeO) articles is conventionally achieved by hot pressing beryllium oxide powder into the desired configuration. A sintering aid such as lithium oxide was often blended with beryllium oxide powder prior to the pressing operation to facilitate the sintering of the beryllium oxide particulates for providing an article of near theoretical density at significantly lower temperatures and pressures than attainable without the use of a sintering aid.
The lithium oxide was preferably introduced into the beryllium oxide powder as lithium hydroxide from an essentially saturated aqueous solution. The concentration of the lithium hydroxide in the aqueous solution was sufficient to coat essentially each beryllium oxide particulate with lithium hydroxide. Normally, lithium hydroxide in the aqueous solution was in a concentration of about 4 to 18 wt. % which provided a lithium hydroxide equivalent to lithium oxide (Li.sub.2 O) of about 2.5 to 11 wt. %, a 10 wt. % addition of which was used for blending purposes. The blending of the beryllium oxide powder with the lithium hydroxide caused the beryllium oxide particulates to be coated with lithium hydroxide. During the hot pressing operation, the water was evaporated from the aqueous solution and the lithium hydroxide converted to lithium oxide which was then combined with the beryllium oxide to form a small amount of lithium beryllate. The presence of this beryllate caused formation of a small amount of a liquid phase (eutectic) at about 870.degree. C. which markedly aided in the densification of the beryllium oxide body.
While the use of lithium hydroxide provided for lower hotpressing temperature and pressures, some shortcomings were present which detracted from the use of this sintering aid. For example, it is believed that a significant amount of the lithium hydroxide becomes depleted locally by leaching or wicking through an evaporation-condensation mechanism during the hot pressing operation. As a result of this depletion, the hot pressing of some configurations such as those that require the use of a closed-end die assembly causes a markedly different visual appearance to occur at the free end of the hot-pressed article as compared to the rest of the article. In addition to such cosmetic effects, it has been found that undesirable density gradients are present throughout the hot pressed article with these den-density gradients apparently being caused by the migration and depletion of the lithium hydroxide. These density gradients are frequently responsible for the rejection of hot-pressed articles of beryllium oxide which significantly increases production costs.